FIG. 5 shows a cross-sectional view of a prior art image sensor (line sensor) in which a-Si PD and a-Si TFT are integrated on a substrate, disclosed in Japanese Laid-open Patent Publication No. 61-89661. In FIG. 5, a gate electrode 2 is disposed on a desired portion of an insulating substrate 1, and a gate insulating film 3 is disposed on the remainder of substrate 1 and on film 3. An undoped a-Si film as an active layer 4 is disposed on a portion of the gate insulating film 3 to cover the gate electrode 2. A P (phosphorus) doped n.sup.+ -a-Si film 5 is disposed on the active layer 4 as a source/drain electrode in an ohmic contact with the active layer 4, and a source/drain electrode 6 is disposed thereon. A channel protection film 7a is disposed in a hollow section of the source/drain electrode 6, and a TFT protection film 7b is disposed thereon. Thus, a-Si TFT 30 for reading out charges stored at a photodiode is constituted by elements 2, 3, 4, 5, 6, 7a and 7b. An undoped a-Si film 9 which constitutes a photodiode is disposed on another portion of the gate insulating film 3, and an electrode material 11b is provided thereon. Reference numeral 10 designates a transparent electrode. Thus, a-Si PD 20 is constituted by elements 9, 10, and 11b. An electrode material 11a is provided for connecting the a-Si PD 20 with the a-Si TFT 30.
The operation of this image sensor is well known. That is, an incident light is converted into an electrical signal by a-Si PD 20 constituted by the electrode material 11b, the a-Si film 9, and the transparent electrode 10, and the electric signal is read out by turning on the gate electrode 2 of TFT.
In the prior art image sensor described above, since the a-Si PD 20 and the a-Si TFT 30 are separately produced, the electrode material 11a is provided for connecting the a-Si PD 20 and the a-Si TFT 30. Therefore, a contact defect may occur at the contact surface of the electrode material 11a and the transparent electrode 10 or a contact surface of the electrode material 11a and the source/drain electrode 6 due to for example, an oxide which is generated in the transparent electrode 10 or in the source/drain electrode 6 furthermore, electrode material 11a may be broken due to the difference in levels of the source/drain electrode 6, the n.sup.+ -a-Si film 5, and the undoped a-Si film 4. Accordingly, the connection defect may occur between the a-Si PD 20 and the a-Si TFT 30, thereby resulting in unstable operating characteristics and a reduction in the yield.
FIG. 6 shows a cross-sectional view of another prior art image sensor in which a-Si PD, a-Si TFT, and matrix wiring for taking signals to the outside are integrated on an insulating substrate, disclosed in "A Technical Report of Institute of Electronics and Communication Engineers ED 83-72 (1983)". In Fig. 6, the same reference numerals designate the same elements as those shown in FIG. 5. Reference numerals 3a and 3b designate a gate insulating film and an inter-wiring insulating film, respectively. A channel protection film 7 is provided for protecting the channel region. A second conductor 8 of matrix wiring section 40 is connected with the source/drain electrode 6 of a-Si TFT through a portion of electrode 6 that is arranged in a direction generally normal to the source/drain electrode.
The operation of this image sensor will be described with reference to FIG. 7. FIG. 7 shows an equivalent circuit diagram of an image sensor of FIG. 6. An incident light is converted into an electric signal in the undoped a-Si film 9, and the electric signal is stored in a floating capacitor between PD and TFT. Next, the gate line G1 is turned on, and the other gate lines are turned off. The electric signals which are obtained by light-electricity conversion and stored at photodiodes PD.sub.1,1 to photodiodes PD.sub.1,64 are transferred to load capacitors C.sub.1 to C.sub.64 through transistors T.sub.1,1 to T.sub.1,64. After turning off the gate line G1, the stored electric charges are read out as an output signal by the scanning of the data lines D.sub.1 to D.sub.64 by an external circuit. Thereafter, the load capacitors C.sub.1 to C.sub.64 are initialized by a reset signal, and the gate line G2 is turned on, and the electric signals which are obtained by light-electricity conversion at photodiodes PD.sub.2,1 to PD.sub.2,64 are transferred to the load capacitors C.sub.1 to C.sub.64. By repeating the above-described operation 27 times, the information for one line (1728 bits/line, 8 dots/mm) can be read out. Thus, the device shown in FIG. 6 operates as an image sensor.
In the prior art image sensor described above, the gate electrode 2 and the second conductor 8 are produced in the same process step. In this case, since the gate insulating film 3a of the a-Si TFT and the inter-wiring insulating film 3b of the matrix wiring section are required to have different thicknesses, these films 3a and 3b are respectively produced by different process steps. Furthermore, since the second conductor 8 is wired below the source/drain electrode 6, the undoped a-Si film 4 and the n.sup.+ -a-Si film 5 cannot be left in the area of the matrix wiring section in order to achieve good coverage by the electrode 6. Therefore, the source/drain electrode 6 is likely to be broken at the changes in levels of the undoped a-Si film 4 and the n.sup.+ -a-Si film 5, thereby resulting in reduction in the yield. Furthermore, at least two process steps are required between the production of the second conductor 8 and the production of the source/drain electrode 6, and therefore an oxide film is likely to form on the second conductor 8 and connection defects are likely to occur between the second conductor 8 and the source/drain electrode 6, thereby resulting in variation in the device characteristics.